Disable plugin from WordPress file(s)

I recently installed a plugin that seems to have brought down the whole of the admin panel. Whichever page I go to, I get “An Unexpected HTTP Error occurred during the API request”.

This started right after installing SexyBookmarks. This particular blog, however, is hosted at a computer where outgoing connections are restricted.

Question 1: which file do I manually edit to disable the plugin in question?

Question 2: given that the blog was still working even without outgoing connections, the only problem I encountered with this setup was when activating the Akismet plugin. Could SexyBookmarks really have done this?